1. Field of the Invention
The present invention relates to a pipe joint structure and a technique for readily joining pipes in a pipe line. More particularly, the invention relates to a novel pipe joint structure for butt joining ends of pipes in a pipe line to be use in transportation of water for hydroelectric power generation, transportation of service water or sewage, and transportation of mineral slurry or corrosive fluid, and also relates to a technique for collectively joining pipes provided with such pipe joint structures in the pipe line.
2, Related Art Statement
Heretofore, as pipe joints for line pipes, (1) welded joints, (2) flange joints, and (3) mechanical joints are recited.
In the case of the welded joint (1), since ends of pipes need to be directly joined at a construction site by welding, this technique required a great deal of skill. Further, since the welded joint needs to be inspected by X rays and repaired by coating, it takes a long time for the joining (about 5 hours per one joint in the case of 800 mm diameter pipes).
The flange joint (2) is unsuitable for buried pipes liable to be exposed to uneven sinking, because the flange joint is more expensive and has lower resistance against bending moment as compared with the welded joint.
The mechanical joint (3) is better than the joints (1) and (2) in that the former has easy to work, and has appropriate elongation and flexibility at the joined portion. However, the mechanical joint has the following defects. That is, the mechanical joint is more costly than the welded joint. The sealingness of the mechanical joint is gradually deteriorated, and fracture of the rubber ring or deassembling of the joint tend to more frequently occur particularly on earthquakes.
FIG. 9 shows a conventional mechanical joint. When an internal pressure P is applied inside pipes 81 and 81 of this joint, the pressure is also ordinarily imparted upon a middle ring (outer cylindrical pipe) 82 and a rubber packing 83. Therefore, the internal pressure acts upon the middle ring 82 over a width equal to an outer width W of the rubber packing 83. Consequently, the joint must be designed to sufficiently withstand such a pressure. Further, in order to also withstand the above internal pressure, metal fixtures for the rubber packing 83 must be tightened by a number of bolts and nuts 84. Owing to this, the structure becomes complicated, resulting in a high cost.
In addition, the joint portion needs to be essentially sealed only at an expansion-contraction gap C in FIG. 9.
Incidentally, the conventional mechanical joint is based on a way of thinking that the joined portion itself freely slides on vibrations in large earthquakes. For this reason, the above width W is made so large that the joined pipes will not be slipped off from the joined portion even upon receipt of great vibrations.
Moreover, since a sliding surface to the rubber packing 83 in the conventional mechanical joint may be brought into contact with the fluid, the sliding surface is covered with an anti-corrosive paint such as an epoxy paint. However, the sliding surface is often rusted through abrasion of the coat film due to vigorous sliding or heaping of earth and sand on the sliding surface. When the rusted portion has gradually spread, the rubber packing is broken by a rusty mass or earth and sand to cause leakage upon receipt of great vibrations such as earthquake.
On the other hand, when the sliding surface to the rubber packing 83 is rusted to make the sliding resistance greater, that part of the joined portion which has a relatively smaller sliding resistance is concentratedly displaced on earthquake, so that the joint may in its turn be disassembled or the like. As mentioned above, even when the width W of the joined portion is made greater, great variations occur in the sliding resistance varies if the sliding surface is rusted. Ultimately, the intended effects cannot be realized.